I. Field of the Invention
This invention relates to an improved method of preparing base stocks for lubricating oil with improved color, thermal and oxidative stability. More particularly, it relates to an improved method of preparing base stocks for lubricating oil with improved color, thermal and oxidative stability by varying the temperatures and pressures of the hydrofinishing process.
II. Background of the Invention
It is well-known that lubricants are susceptible to deterioration by oxidation. Lubricants can be attacked by oxygen or air at high temperatures to form heavy dark viscous sludges, varnish and resins. Such deterioration reduces a lubricant's effectiveness to perform its required task. Accompanying the deterioration of lubricants by oxidation is the resultant corrosion of the metal surfaces for which such lubricants were designed to protect. Acids develop from the sludges and resins which are corrosive enough to destroy most metals. In addition, increased demands on lubricants brought about by newer and larger engines and other rotating or moving equipment, operating at increasing temperatures, pressures and speeds dictate the need for improvement in a lubricant's resistance to deterioration by oxidation. A lubricant's color, thermal and oxidative stability reflect the lubricant's resistance to oxidation.
This invention relates to an improved method for preparing base stocks for lubricating oil using a hydrogen treatment technique. The three most common hydrogen treatment techniques associated with lubricating oil production, as described in U.S. Pat. No. 3,915,841 to Murphy, Jr., et al., are hydrocracking, hydrotreating and hydrofinishing.
Hydrocracking is an extremely severe hydrogen treatment, usually conducted at comparatively high temperatures requiring employment of a catalyst having substantial cracking activity, e.g., an activity index (A.I.) greater than 40 and generally greater than 60. This type of process is conducted to effect extensive and somewhat random severing of carbon-to-carbon bonds resulting in an substantial overall reduction in molecular weight and boiling point of the treated material. Thus, for example, hydrocracking processes are generally employed to effect an extremely high conversion, e.g., 90% by volume, to materials boiling below the boiling range of the feedstock or below a designated boiling point. Usually a hydrocracking process is employed to produce a product boiling predominantly, if not completely, below about 600.degree. F. to 650.degree. F. Most frequently, this type of process is employed to convert higher boiling hydrocarbons with products boiling in the furnace oil and naphtha range. When applied in connection with lubricating oils, hydrocracking processes produce only a minor quantity of materials boiling in the lubricating oil range, i.e., 625.degree. F. to 650.degree. F., to the extent, at times, the production of a lubricating oil is merely incidental to the production of naphtha and furnace oil.
As distinguished from hydrocracking and hydrofinishing, hydrotreating is a processing technique significantly more severe than hydrofinishing although substantially less severe than hydrocracking. The catalyst required in a hydrotreating process must possess cracking activity and generally possess a particular type of activity termed "ring scission activity." A hydrotreating process effects a substantial molecular rearrangement as compared to the hydrofinishing but does not effect the extensive and somewhat random breakdown in molecules effected in hydrocracking.
On the other end of the spectrum, hydrofinishing is a mild hydrogen treatment process employing a catalyst having substantially no cracking activity. This fixed-bed catalytic hydrogenation process effects removal of contaminants such as color forming bodies and a reduction of minor quantities of sulfur, oxygen, and nitrogen compounds. Unlike hydrocracking or hydrotreating, hydrofinishing does not saturate aromatics, nor break carbon-carbon bonds. As a general rule, hydrofinishing is employed in lieu of older techniques of acid and clay contacting for the purpose of improving color, odor, thermal and oxidative stability of lubricating oil base stocks. The operating conditions of the hydrofinishing process are a function of the feedstock composition, catalyst type and product specifications.
Conventional hydrofinishing temperatures and pressures have nominally been in the 450.degree. F. to 500.degree. F. and 400 psi to 700 psi range. However, conventional hydrofinishing conditions have not produced lubricating oil base stocks with satisfactory color, thermal and oxidative stability needed for operating under conditions of steadily increasing temperatures and pressures. Accordingly, this invention provides a process whereby the color, thermal and oxidative stability of lubricating oil stocks is improved. Surprisingly, these improvements were obtained by increasing the temperature of the hydrofinishing process.